U.S. patent number 10,510,830 [Application Number 16/120,300] was granted by the patent office on 2019-12-17 for n-type polysilicon crystal, manufacturing method thereof, and n-type polysilicon wafer.
This patent grant is currently assigned to Sino-American Silicon Products Inc.. The grantee listed for this patent is Sino-American Silicon Products Inc.. Invention is credited to Yuan-Hsiao Chang, Sung-Lin Hsu, Ying-Ru Shih, Bo-Kai Wang, Ching-Hung Weng, Cheng-Jui Yang, Yu-Min Yang, Wen-Huai Yu.
United States Patent |
10,510,830 |
Weng , et al. |
December 17, 2019 |
N-type polysilicon crystal, manufacturing method thereof, and
N-type polysilicon wafer
Abstract
An N-type polysilicon crystal, a manufacturing method thereof,
and an N-type polysilicon wafer are provided. The N-type
polysilicon crystal has a slope of resistivity and a slope of
defect area percentage. When the horizontal axis is referred to
solidified fraction and the vertical axis is referred to
resistivity presented by a unit of Ohmcm (.OMEGA.cm), the slope of
resistivity is 0 to -1.8 at the solidified fraction of 0.25 to 0.8.
When the horizontal axis is referred to solidified fraction and the
vertical axis is referred to defect area percentage (%), the slope
of defect area percentage is less than 2.5 at the solidified
fraction of 0.4 to 0.8.
Inventors: |
Weng; Ching-Hung (Hsinchu,
TW), Yang; Cheng-Jui (Hsinchu, TW), Yang;
Yu-Min (Hsinchu, TW), Chang; Yuan-Hsiao (Hsinchu,
TW), Wang; Bo-Kai (Hsinchu, TW), Yu;
Wen-Huai (Hsinchu, TW), Shih; Ying-Ru (Hsinchu,
TW), Hsu; Sung-Lin (Hsinchu, TW) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sino-American Silicon Products Inc. |
Hsinchu |
N/A |
TW |
|
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Assignee: |
Sino-American Silicon Products
Inc. (Hsinchu, TW)
|
Family
ID: |
65431892 |
Appl.
No.: |
16/120,300 |
Filed: |
September 2, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20190096987 A1 |
Mar 28, 2019 |
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Foreign Application Priority Data
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Sep 25, 2017 [TW] |
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106132853 A |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B
11/00 (20130101); H01L 29/04 (20130101); H01L
29/045 (20130101); H01L 31/182 (20130101); C30B
29/06 (20130101); H01L 31/03682 (20130101); H01L
29/167 (20130101); C30B 28/06 (20130101); H01L
29/32 (20130101); Y02P 70/521 (20151101); Y02E
10/546 (20130101) |
Current International
Class: |
H01L
29/32 (20060101); H01L 29/167 (20060101); H01L
29/04 (20060101); H01L 31/0368 (20060101); C30B
29/06 (20060101); C30B 11/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102312279 |
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Jan 2012 |
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CN |
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102560641 |
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Jul 2012 |
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CN |
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104532345 |
|
Apr 2015 |
|
CN |
|
201708634 |
|
Mar 2017 |
|
TW |
|
Other References
Florian Schindler et al., "High-Efficiency Multicrystalline Silicon
Solar Cells:Potential of n-Type Doping" IEEE Journal of
Photovoltaics, vol. 5, No. 6, Nov. 2015, pp. 1571-1579. cited by
applicant .
Florian Schindler et al., "The Potential of Multicrystalline N-Type
Silicon for High Efficiency Solar Cells" 29th European PV Solar
Energy Conference and Exhibition, Sep. 22-26, 2014, pp. 1-5. cited
by applicant.
|
Primary Examiner: Withers; Grant S
Attorney, Agent or Firm: JCIPRNET
Claims
What is claimed is:
1. An N-type polysilicon crystal, wherein: a resistivity of the
N-type polysilicon crystal has a slope when graphed, of which a
horizontal axis is referred to as a solidification fraction and a
vertical axis is referred to as the resistivity presented by a unit
of Ohm-cm (.OMEGA.cm), and the slope of resistivity is between 0 to
-1.8 at a first solidification fraction value between 0.25 to 0.8;
and a defect area percentage of the N-type polysilicon crystal has
a slope when graphed, of which the horizontal axis is referred to
as the solidification fraction and the vertical axis is referred to
as the defect area percentage (%), and the slope of defect area
percentage is less than 2.5 at a second solidification fraction
value between 0.4 to 0.8.
2. The N-type polysilicon crystal of claim 1, wherein an average
value of a minority carrier lifetime of the N-type polysilicon
crystal measured via a .mu.-PCD method is greater than 20
.mu.s.
3. The N-type polysilicon crystal of claim 1, wherein the N-type
polysilicon crystal is doped with gallium and phosphorous, a doping
amount of the gallium is 0.3 ppma to 3 ppma, a doping amount of the
phosphorous is 0.02 ppma to 0.2 ppma, and an atomic ratio of the
gallium with respect to the phosphorous is between 10 and 20.
4. The N-type polysilicon crystal of claim 1, wherein the N-type
polysilicon crystal comprises an ingot, a brick, or a wafer.
5. The N-type polysilicon crystal of claim 1, wherein a crystal
orientation of the N-type polysilicon crystal at least comprises
{111}, {112}, {113}, {315}, and {115}.
6. The N-type polysilicon crystal of claim 1, wherein the defect
area percentage of the N-type polysilicon crystal is less than 2%
at the solidified fraction of 0.4 to 0.8.
7. The N-type polysilicon crystal of claim 1, wherein when an
oxygen content in the N-type polysilicon crystal is in a range of
greater than or equal to 5 ppma, a carbon content of a
corresponding location thereof is greater than or equal to 4
ppma.
8. The N-type polysilicon crystal of claim 7, wherein the N-type
polysilicon crystal comprises a plurality of silicon crystal grains
grown along a crystal growth direction, wherein an average crystal
grain size of the silicon crystal grains in the crystal growth
direction and a resistivity of the N-type polysilicon crystal have
opposite changing trends.
9. The N-type polysilicon crystal of claim 8, wherein the average
crystal grain size is less than or equal to 1.3 cm.
10. An N-type polysilicon wafer obtained by cutting the N-type
polysilicon crystal of claim 1, wherein an average value of a
minority carrier lifetime of the N-type polysilicon wafer measured
via a .mu.-PCD method is 2 .mu.s to 5 .mu.s.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application
serial no. 106132853, filed on Sep. 25, 2017. The entirety of the
above-mentioned patent application is hereby incorporated by
reference herein and made a part of this specification.
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an N-type polysilicon crystal growth
technique, and more particularly, to an N-type polysilicon crystal,
a manufacturing method thereof, and an N-type polysilicon
wafer.
Description of Related Art
A solar cell is an optoelectronic device generating electrical
energy from absorbed sunlight via photovoltaic effect. Currently,
the material of the solar cell is mainly based on silicon
materials, such as single-crystal silicon, polysilicon, or
amorphous silicon.
By using polysilicon as a raw material for a solar cell, the cost
is significantly less than the cost of a solar cell using single
crystal silicon manufactured via an existing Czochralski method
(CZ) method and floating zone (FZ) method.
In general, polysilicon crystal growth is casting-based, and P-type
polysilicon is mainly applied in the solar cell. However, P-type
polysilicon does not compete well against single crystals in the
market and fails to maintain market share because of low conversion
efficiency. Therefore, development of an N-type polysilicon having
significantly higher conversion efficiency is urgently needed.
However, resistance of current N-type polysilicon ingot is widely
distributed, which leads to an issue of low production yield.
SUMMARY OF THE INVENTION
The invention provides an N-type polysilicon crystal having
concentrated resistance distribution and great crystal quality.
The invention further provides an N-type polysilicon wafer having
longer minority carrier lifetime.
The invention further provides a manufacturing method of an N-type
polysilicon crystal that can produce a crystal having concentrated
resistance distribution and great quality.
The N-type polysilicon crystal of the invention has a slope of
resistivity and a slope of defect area percentage. When the
horizontal axis unit is referred to a solidified fraction and the
vertical axis is referred to the resistivity presented by a unit of
Ohmcm (.OMEGA.cm), the slope of resistivity is 0 to -1.8 at the
solidified fraction of 0.25 to 0.8. When the horizontal axis is
referred to the solidified fraction and the vertical axis is
referred to the defect area percentage (%), the slope of defect
area percentage is less than 2.5 at the solidified fraction of 0.4
to 0.8.
In an embodiment of the invention, the average value of minority
carrier lifetime of the N-type polysilicon crystal measured via a
.mu.-PCD (microwave photoconductivity decay) method is greater than
20 .mu.s.
In an embodiment of the invention, the N-type polysilicon crystal
is doped with gallium and phosphorous, and the doping amount of
gallium is such as 0.3 ppma to 3 ppma, the doping amount of
phosphorous is such as 0.02 ppma to 0.2 ppma, and the atomic ratio
of gallium with respect to phosphorous is such as between 10 and
20.
In an embodiment of the invention, the N-type polysilicon crystal
includes an ingot, a brick, or a wafer.
In an embodiment of the invention, a crystal orientation of the
N-type polysilicon crystal at least includes {111}, {112}, {113},
{315}, and {115}.
In an embodiment of the invention, the defect area percentage of
the N-type polysilicon crystal is less than 2% at the solidified
fraction of 0.4 to 0.8.
In an embodiment of the invention, when the oxygen content of the
N-type polysilicon crystal is in the range of greater than or equal
to 5 ppma, the carbon content of the corresponding location thereof
is greater than or equal to 4 ppma.
In an embodiment of the invention, the N-type polysilicon crystal
includes a plurality of crystal grains grown along a crystal growth
direction, wherein the average crystal grain size of the silicon
crystal grains in the crystal growth direction and the resistivity
of the N-type polysilicon crystal have opposite changing
trends.
In an embodiment of the invention, the average crystal grain size
can be less than or equal to 1.3 cm.
The N-type polysilicon crystal wafer of the invention is obtained
via cutting the N-type polysilicon crystal described above, and the
average value of minority carrier lifetime measured via a .mu.-PCD
(microwave photoconductivity decay) method is 2 .mu.s to 5
.mu.s.
In the manufacturing method of the N-type polysilicon crystal of
the invention, an N-type polysilicon crystal is grown using a
crystal growth furnace of a directional solidification system
(DSS), wherein a crucible in the crystal growth furnace contains a
silicon material and a dopant, a height of the N-type polysilicon
crystal is H, and the dopant is located in a region of 0.1 H to 0.3
H from a bottom portion of the crucible.
In another embodiment of the invention, the dopant includes a
particle, a doping sheet, or a combination thereof.
In another embodiment of the invention, the dopant includes gallium
and phosphorus.
In another embodiment of the invention, the doping amount of
gallium is about 0.3 ppma to 3 ppma, the doping amount of
phosphorous is about 0.02 ppma to 0.2 ppma, and the atomic ratio of
gallium with respect to phosphorous is between about 10 and 20.
In another embodiment of the invention, the silicon material can
include a waste wafer.
In another embodiment of the invention, the dopant is such as a
particle, and the following steps can be further included before
the N-type polysilicon crystal is grown. A portion of a silicon
material is covered by the waste wafer, then the particle is placed
on the waste wafer, and then the particle is surrounded and covered
by the remainder of the silicon material.
In another embodiment of the invention, the dopant is, for
instance, a particle and a doping sheet, and the following can be
further included before the N-type polysilicon crystal is grown. A
portion of a silicon material is covered using the waste wafer,
then the particle is placed on the waste wafer, and then the
particle is covered by another waste wafer. Next, the doping wafer
is placed on the another waste wafer on the particle, and the
remainder of the silicon material is added.
Based on the above, in the invention, during the process of growing
the N-type polysilicon crystal in a DSS crystal growth furnace, the
dopant is placed in a region of a specific range, and therefore an
N-type polysilicon crystal having concentrated resistance
distribution and great quality can be manufactured. In addition, an
N-type polysilicon wafer having great quality and longer minority
carrier lifetime is obtained via cutting such N-type polysilicon
crystal. Thus, the N-type polysilicon wafer can be made in a solar
cell having better conversion efficiency than a solar cell prepared
by a P-type polysilicon wafer.
In order to make the aforementioned features and advantages of the
disclosure more comprehensible, embodiments accompanied with
figures are described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification. The drawings illustrate
embodiments of the invention and, together with the description,
serve to explain the principles of the invention.
FIG. 1 is a schematic view illustrating preparation of an N-type
polysilicon crystal according to a first embodiment of the
invention.
FIG. 2 is a line diagram illustrating a relationship of resistivity
with respect to solidified fraction of the polysilicon crystals of
experimental examples 1 to 2 and comparative example 2.
FIG. 3 is a line diagram illustrating minority carrier lifetime of
the polysilicon crystal of experimental examples 1 to 2 and
comparative example 1.
FIG. 4 is a line diagram illustrating minority carrier lifetime of
polysilicon wafers prepared by the polysilicon crystals of
experimental examples 1 to 2 and comparative example 1.
FIG. 5 is a line diagram illustrating relationship of defect area
percentage with respect to solidified fraction of polysilicon
wafers prepared by the polysilicon crystals of experimental
examples 1 to 2 and comparative example 1.
FIG. 6 is a line diagram illustrating relationship of carbon/oxygen
content with respect to solidified fraction of the polysilicon
column of experimental example 1.
FIG. 7 is a line diagram illustrating relationship of average
crystal grain size with respect to solidified fraction of the
polysilicon crystals of experimental examples 1 to 2 and
comparative example 1.
FIG. 8 is a comparison chart illustrating conversion efficiency of
solar cells applying polysilicon wafers prepared by the polysilicon
crystals of experimental example 1, comparative example 1, and
comparative example 3.
DESCRIPTION OF THE EMBODIMENTS
The text above summarily described the figures of the invention,
and better understanding can be attained via the detailed
description of the invention below. Other technical features and
advantages of the subject matters of the claims of the invention
are described below. Those having ordinary skill in the art of the
invention should understand that, the concepts and specific
embodiments disclosed below can be used as a basis for modifying or
designing other structures or processes to implement the same
object as the invention. Those having ordinary skill in the art of
the invention should also understand that, such equivalent
construction cannot depart from the spirit and scope of the
invention provided in the accompanying claims.
The disclosure below provides numerous different embodiments or
examples for implementing different features of the invention.
Descriptions of specific examples of devices and configurations are
provided below, to simplify the disclosure of the invention. Of
course, these are only examples, and are not intended to limit the
scope and application of the invention. Moreover, for clarity, the
relative thicknesses and locations of regions or structural devices
may be reduced or enlarged. Moreover, similar or same reference
numerals in each of the figures tend to indicate the presence of
similar or same devices or features. Similar reference numerals in
the figures represent similar devices and descriptions thereof are
omitted.
FIG. 1 is a schematic view illustrating preparation of an N-type
polysilicon crystal according to a first embodiment of the
invention.
FIG. 1 shows a crucible 102 inside a crystal growth furnace 100 of
a directional solidification system (DSS), and other components
thereof are similar to those of known or existing DSS crystal
growth furnaces.
In the present embodiment, the crucible 102 contains a silicon
material 104 and dopants 106a and 106b. If the height of the N-type
polysilicon crystal grown via DSS is H, then the locations of the
dopants 106a and 106b need to be in a region of 0.1 H to 0.3 H from
a bottom portion 102a of the crucible 102. In FIG. 1, the dopant
106a is a particle and the dopant 106b is a doping sheet, but in
the invention, the dopants can respectively be particles or
sheets.
In some embodiments, a particulate dopant 106a is used, such as a
gallium (Ga) dopant. Since the particulate dopant 106a is readily
volatilized and is in a particulate shape, a waste wafer 108 can be
used to cover the silicon material 104 in the region from 0.1 H to
0.3 H described above, and then the dopant 106a can be surrounded
and covered by the remainder of the silicon material 104. Moreover,
after using the waste wafer 108 to cover the particulate dopant
106a, another sheet dopant 106b (e.g., phosphorus-doped silicon
sheet) can be placed thereafter, and then the silicon material 104
can be added. The configuration of the dopants 106a and 106b can be
modified according to the form thereof and is not limited to the
configurations described above. The effect of reducing
volatilization of the dopants 106a and 106b can be achieved as long
as the locations of the dopants 106a and 106b are ensured to be in
the region of 0.1 H to 0.3 H. Moreover, the waste wafers can also
be regarded as a portion of the silicon material 104 for crystal
growth.
In an embodiment, the dopants 106a and 106b can include gallium and
phosphorous, as long as the placement locations thereof satisfy the
above ranges. From the standpoint of achieving concentrated
resistance distribution and great quality for the N-type
polysilicon crystal, in the dopants 106a and 106b, the doping
amount of gallium is such as 0.3 ppma to 3 ppma, and the doping
amount of phosphorous is such as 0.02 ppma to 0.2 ppma, and the
atomic ratio of gallium with respect to phosphorous is such as in
between 10 and 20.
In an embodiment, the crystal growth process of DSS includes
configuring the silicon material 104 and the dopants 106a and 106b
as shown in FIG. 1, then performing heating to completely melt the
silicon material 104 inside the crucible 102 into molten silicon,
and then a directional solidification process is performed to cool
the crucible 102 and the silicon material 104 inside, such that
polysilicon crystal grains are gradually grown along a crystal
growth direction V to form an N-type polysilicon crystal.
In the above embodiments, the devices (such as carbon fiber,
insulating material, or graphite plate) inside the crystal growth
furnace 100 produces a carbon element due to high temperature, and
therefore in the case that the crucible opening is not covered by a
top cover plate (not shown), the carbon element produced in the
environment enters the molten silicon, and as a result the N-type
polysilicon crystal manufactured is expected to have a higher
carbon content.
According to the preparation method of the first embodiment, the
N-type polysilicon crystal grown using DSS has a slope of
resistivity and a slope of defect area percentage. When the
horizontal axis is referred to solidified fraction and the vertical
axis is referred to resistivity presented by unit of Ohmcm
(.OMEGA.cm), the slope of resistivity is 0 to -1.8 at the
solidified fraction of 0.25 to 0.8. When the horizontal axis unit
is referred to solidified fraction and the vertical axis unit is
referred to defect area percentage (%), the slope of defect area
percentage is less than 2.5 at the solidified fraction of 0.4 to
0.8.
In the present disclosure, "solidified fraction" in the N-type
polysilicon crystal solidification process refers to the ratio of
the height of the solidified portion of the N-type polysilicon
crystal along the crystal growth direction V thereof with respect
to the total height of the silicon crystal. Portions of the crystal
being solidified in shorter time has a smaller solidified fraction,
while portions of the crystal being solidified in longer time has a
greater solidified fraction. As such, a solidified fraction of 0
represents the bottom portion of the N-type polysilicon crystal,
while a solidified fraction of 1.0 represents the top portion of
the N-type polysilicon crystal.
In an embodiment, the average value of minority carrier lifetime of
the N-type polysilicon crystal measured via a .mu.-PCD method is
greater than 20 .mu.s. Moreover, the N-type polysilicon crystal can
be doped with gallium and phosphorous, and the doping amount of
gallium is such as between 0.3 ppma and 3 ppma, the doping amount
of phosphorous is such as between 0.02 ppma and 0.2 ppma, and the
atomic ratio of gallium with respect to phosphorous is such as
between 10 and 20. For instance, a smaller atomic ratio of gallium
with respect to phosphorous indicates a smaller resistivity of the
grown N-type polysilicon crystal. On the other hand, a greater
atomic ratio of gallium with respect to phosphorous indicates a
greater resistivity of the grown N-type polysilicon crystal.
Moreover, the crystal orientations of the above-mentioned N-type
polysilicon crystal at least includes {111}, {112}, {113}, {315},
and {115}, but the invention is not limited thereto. Moreover, the
N-type polysilicon crystal of the present embodiment has a defect
area percentage less than 2% in the solidified fraction range of
0.4 to 0.8. The N-type polysilicon crystal generally includes an
ingot, a brick, or a wafer.
The oxygen content of portions of the N-type polysilicon crystal at
the solidified fraction of 0 to 0.15 can be greater than or equal
to 5 ppma, such as between 5 ppma and 20 ppma, preferably between 5
ppma and 10 ppma. Moreover, since a top cover plate is not used in
the crystal growth process, when the oxygen content of the N-type
polysilicon crystal is in the range of greater than or equal to 5
ppma, the carbon content at the corresponding location thereof can
be greater than or equal to 4 ppma such as between 5 ppma and 20
ppma, preferably between 6 ppma and 13 ppma. Moreover, it can be
seen that the average crystal grain size of silicon crystal grains
along the crystal growth direction V in the N-type polysilicon
crystal and the resistivity of the N-type polysilicon crystal have
opposite changing trends. In an embodiment, the average crystal
grain size can be less than or equal to 1.3 cm.
Several experiments are provided below to verify the efficacy of
the disclosure. However, the scope of the disclosure is not limited
to the following experiments.
Experimental Example 1
A silicon material and dopants (gallium particle and phosphorous
doping sheet) were placed in a graphite crucible as shown in FIG.
1, and the location of the dopant was in a region of 0.25 H from
the bottom portion of the crucible. The doping amount of gallium
was 1.953 ppma, and the doping amount of phosphorous was 0.180
ppma. The atomic ratio of gallium with respect to phosphorous was
10.85.
The temperature was raised over 1414.degree. C. without the top
cover plate being placed on the crucible, and the silicon material
begun to be melted. When heated to 1500.degree. C. to 1570.degree.
C., the silicon material was completely melted to a molten silicon,
and then crystal growth was performed by controlling the
temperature. Initial temperature of crystal growth was set to
1385.degree. C. to 1430.degree. C., and the final temperature was
set to 1385.degree. C. to 1400.degree. C. After the crystal growth
was complete, annealing and cooling processes were performed in
order, and N-type polysilicon crystal can be obtained.
Experimental Example 2
The same crystal growth steps and dopant placement location as
experimental example 1 were used, but the doping amounts of gallium
and phosphorous were respectively changed to 0.632 ppma and 0.044
ppma. The atomic ratio of gallium with respect to phosphorous was
14.36. An N-type polysilicon crystal can be obtained.
Comparative Example 1
The same crystal growth steps as experimental example 1 were used,
except that only boron was doped (doping amount: 0.183 ppma), and
therefore a P-type polysilicon crystal was obtained.
Comparative Example 2
Relevant experimental results in "The Potential of Multicrystalline
N-Type Silicon for High Efficiency Solar Cells" published in the
29th European PV Solar Energy Conference and Exhibition in
Amsterdam, Holland on 22 to 24 Sep. 2014 by Schindler et al. were
used as comparative example 2. In particular, the N-type dopant was
phosphorous.
Comparative Example 3
Relevant experimental results in pages 1571 to 1579 of
"High-Efficiency Multicrystalline Silicon Solar Cells: Potential of
n-Type Doping" in IEEE Journal of Photovoltaics, Vol. 5, No. 6
published in November 2015 by Schindler et al. were used as
comparative example 3. In particular, the doping amount of the
N-type dopant was 7.times.10.sup.15 cm.sup.-3 to 8.times.10.sup.15
cm.sup.3, i.e., 0.14 ppma to 0.16 ppma.
<Characterization>
1. Resistivity: a side surface of a polysilicon crystal was
detected via a non-contact resistance testing machine, and the
average value of the measured values at four sides of the portions
of the polysilicon crystal corresponding to each of the solidified
fractions can be regarded as the resistivity of the corresponding
solidified fraction. The non-contact resistance measurement method
included introducing a fixed-frequency alternating current on a
transmission coil. When the magnetic field generated by the coil
approached a test object, an Eddy current occurred to the test
object, and the value of the Eddy current and resistivity were
inversely proportional. Accordingly, the resistivity of the test
object can be obtained.
2. Minority carrier lifetime of polysilicon crystal: a relationship
curve of the minority carrier lifetime of a polysilicon crystal
with respect to solidified fraction was measured using a .mu.-PCD
method.
3. Minority carrier lifetime of polysilicon wafer: a polysilicon
crystal was cut into a number of wafers in a crystal growth
direction (V), and then a minority carrier lifetime testing machine
was used to measure the relationship curve between the minority
carrier lifetime of the polysilicon wafer and solidified fraction.
The thickness of the wafer was not limited as long as it is
acceptable for the wafer sorter inside the equipment, and the
thicknesses of all test pieces had to be identical.
4. Defect area percentage: a polysilicon crystal was cut into a
number of test pieces in a crystal growth direction (V), and then
the test pieces were detected using a photoluminescence (PL)
machine. As such, light having an energy higher than the energy
bandgap of the semiconductor material was irradiated on the test
pieces to generate fluorescence emitted by carrier transition and
recombination behaviors, and then a defect location was determined
according to the fluorescence spectrum via a measurement system to
calculate the relationship between defect area percentage and
solidified fraction.
5. Carbon content of polysilicon crystal: a polysilicon crystal was
cut into a number of test pieces in a crystal growth direction (V),
and then the carbon content at nine different locations on each
test piece was measured using an FTIR measuring equipment with
reference to SEMI MF 1391-0704 standard measurement specifications,
and lastly an average value of the individual carbon content at the
nine locations on the test piece was calculated, and the total
average value from five repeated measurements was used as the
carbon content of the test piece.
6. Oxygen content of polysilicon crystal: same as the measuring
method of carbon content above.
7. Average crystal grain size: a polysilicon crystal was cut into a
number of test pieces in the crystal growth direction (V), and then
the relationship between average crystal grain size and solidified
fraction was measured according to the ASTM E112-10 standard
measurement specification.
First, a measurement of resistivity was performed on the
polysilicon crystals of experimental examples 1 to 2 according to
the characterization method above, and the results are shown
together with the data of comparative example 2 in FIG. 2. It can
be seen from FIG. 2 that, the resistivity of experimental example 1
is between 1.5 .OMEGA.cm and 3 .OMEGA.cm, the resistivity of
experimental example 2 is between 8 .OMEGA.cm and 9 .OMEGA.cm, and
the resistivity of comparative example 2 is between 0.5 .OMEGA.cm
and 3.5 .OMEGA.cm. If the horizontal axis is referred to solidified
fraction and the vertical axis is referred to resistivity presented
by unit of Ohmcm (.OMEGA.cm), then at the solidified fraction of
0.25 to 0.8, the slope of resistivity of experimental example 1 is
-1.63, the slope of resistivity of experimental example 2 is about
-0.3, and the slope of resistivity of comparative example 2 is less
than -1.8, and is -2.5. Therefore, in comparison to the N-type
polysilicon crystal of comparative example 2, the range of
resistivity of experimental examples 1 to 2 is more concentrated
than in comparative example 2, i.e., the slopes of resistivity of
experimental examples 1 to 2 are gentler.
Next, a measurement of minority carrier lifetime was performed on
the polysilicon crystals of experimental examples 1 to 2 and
comparative example 1, and the results are shown in FIG. 3. It can
be seen from FIG. 3 that, the crystal minority carrier lifetimes of
experimental examples 1 to 2 are both 20 .mu.s or greater, and the
maximum value of the crystal minority carrier lifetime of
experimental example 1 reaches 40 .mu.s or more. In comparison, the
minority carrier lifetime of the P-type polysilicon crystal of
comparative example 1 is less than 10 .mu.s.
The results of the minority carrier lifetimes of the polysilicon
wafers of experimental examples 1 to 2 and comparative example 1
are shown in FIG. 4. It can be obtained from FIG. 4 that, the wafer
minority carrier lifetime of experimental example 1 is 3 .mu.s to
4.2 .mu.s, and the wafer minority carrier lifetime of experimental
example 2 is 2.5 .mu.s to 3 .mu.s. In comparison, the P-type
polysilicon wafer minority carrier lifetime of comparative example
1 is about 1.6 .mu.s. Therefore, the average value of the minority
carrier lifetime of experimental examples 1 to 2 of the invention
is in the range of 2 .mu.s to 5 .mu.s, and therefore a longer wafer
minority carrier lifetime than comparative example 1 is
achieved.
Next, a defect measurement was performed on the polysilicon wafers
of experimental examples 1 to 2 and comparative example 1, and the
results are shown in FIG. 5. It can be seen from FIG. 5 that, in
experimental examples 1 to 2 with the solidified fraction of 0.4 to
0.8, the defect area percentages are both less than 2%, and the
average defect area percentage is less than 1.5%. In comparison,
the defect area percentage of comparative example 1 reaches about
2.5%. When the horizontal axis is referred to solidified fraction
and the vertical axis is referred to defect area percentage (%),
the slope of defect area percentage of experimental example 1 at
the solidified fraction of 0.4 to 0.8 is about 2.25, and the slope
of defect area percentage of experimental example 2 at the
solidified fraction of 0.4 to 0.8 is about 0.25. Therefore, the
slopes of defect area percentage of experimental examples 1 to 2 of
the invention are both less than 2.5. In comparison, in comparative
example 1 with the solidified fraction of 0.4 to 0.8, the slope of
defect area percentage is about 4. Therefore, in comparison to the
P-type polysilicon crystal of comparative example 1, the defect
area percentages of experimental examples 1 to 2 are less, and the
slopes of defect area percentage are gentler (smaller), and
therefore the overall brick quality is better than the P-type
polysilicon crystal of comparative example 1.
The results of the carbon content and the oxygen content of the
polysilicon column are shown in FIG. 6 and Table 1 below.
TABLE-US-00001 TABLE 1 Solidified fraction Oxygen content (ppma)
Carbon content (ppma) 0.11 6.172 7.015 0.22 3.44 9.518 0.34 2.379
10.388 0.46 1.442 6.351 0.57 1.274 11.699 0.69 0.917 7.244 0.80
0.407 10.362 0.92 0.244 8.539 1.00 0.238 8.035
It can be seen from FIG. 6 and Table 1 that, the carbon content of
the N-type polysilicon crystal of experimental 1 is greater than 4
ppma when the oxygen content thereof is greater than 5 ppma.
Moreover, it can be seen from FIG. 6 that the oxygen content has a
reverse changing trend in the crystal growth direction (V). That
is, in the crystal growth direction V, portions of the crystal
being solidified in shorter time (i.e., being solidified sooner)
has higher oxygen content, and portions of the crystal being
solidified in longer time (i.e., being solidified later) has lower
oxygen content. In other words, a higher solidified fraction
indicates lower oxygen content.
Lastly, measurement of particle size was performed on experimental
examples 1 to 2 and comparative example 1, and the results are
shown in FIG. 7. It can be seen from FIG. 7 that, the average
crystal grain sizes of silicon crystal grains in the crystal growth
direction of experimental examples 1 to 2 and comparative example 1
are all less than 1.3 cm, and as the solidified fraction is
increased, the crystal grain size tends to be increased. Therefore,
referring to the results of resistivity described above, a
conclusion that the average crystal grain size of silicon crystal
grains in the crystal growth direction V and the resistivity of the
N-type polysilicon crystal have opposite changing trends can be
drawn.
Application Example
The N-type polysilicon wafers formed by cutting the N-type
polysilicon crystals of experimental example 1 and comparative
example 1 were made into solar cells for testing via the same solar
cell manufacturing process in the literature of comparative example
3. The solar cell manufacturing process includes: (1) performing an
etching (or texturing) process on a front surface of the wafer; (2)
performing a masking process shielding the back surface of the
wafer; (3) performing a boron diffusion process (890.degree. C.; 1
hour); (4) performing a step of removing the mask on the back
surface of the wafer; (5) performing a step of forming a tunnel
oxide layer of about several Angstroms on the back surface of the
wafer; (6) performing a step of depositing a 15 nm-thick
phosphorous-doped silicon layer on the tunnel oxide layer; (7)
performing an annealing process (800.degree. C.; 1 hour); (8)
performing a passivation treatment on the front surface of the
wafer; and (9) performing a metallization process.
Next, I-V measurement was performed on the solar cells for testing,
and the results are shown together with the conversion efficiency
in the literature of comparative example 3 in FIG. 8. It can be
seen from FIG. 8 that, the N-type polysilicon wafer of experimental
example 1 of the invention has a conversion efficiency reaching
21.9% which is significantly better than the results of 18.4% of
the P-type polysilicon wafer of comparative example 1 and 19.6% of
the N-type polysilicon wafer of comparative example 3. Therefore,
it can be known that the quality of the N-type polysilicon wafer of
experimental example 1 is better than that of the N-type
polysilicon wafer of comparative example 3.
Based on the above, the N-type polysilicon crystal of the invention
has a gentler slope of resistivity and slope of defect area
percentage, and the minority carrier lifetime is also increased.
Therefore, great crystal quality can be achieved, and a solar cell
having good conversion efficiency can be manufactured from such
N-type polysilicon crystal. Moreover, in the invention, during the
process of growing the N-type polysilicon crystal by a DSS crystal
growth furnace, the dopant can be placed in a region of a specific
range, so as to manufacture the N-type polysilicon crystal having
concentrated resistance distribution and great quality described
above.
Although the invention has been described with reference to the
above embodiments, it will be apparent to one of ordinary skill in
the art that modifications to the described embodiments may be made
without departing from the spirit of the invention. Accordingly,
the scope of the invention is defined by the attached claims not by
the above detailed descriptions.
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